Alien Supply

Once an Alien base is constructed, it is resupplied on a regular basis (approximately every 4 days + 4 hours - or 100 hours) by special supply vessels. If one of these vessels is detected while landing then it is certain that an Alien base is nearby. Once landed, Supply Ships remain on the ground for 33⅓ hours or 1 day, 9 hours and 20 minutes.

Note that supply ships on supply missions are always at a speed that makes them impossible to shoot down by Interceptors, even shortly before landing or after taking off. This means they can only be attacked on the ground unless new aircraft are researched and built. But because supply ships always land, it would be a waste of Elerium to shoot them down. Your new aircraft will need some for the interception, yet the Elerium in the supply ship is usually destroyed by shooting it down.

Piracy time. You may want to let the Alien base exist, just for millions of dollars plundered from the Supply "Blimps". Arrrrrr, avast ye Space-Lubbers!